Closing your swimming pool properly is essential to protect it from damage, algae growth, and chemical imbalances during the off-season. Understanding what chemicals to use to close your pool ensures a smooth and trouble-free opening when warm weather returns. In this comprehensive guide, we walk you through each step, why it matters, and how to apply the best pool closing chemicals effectively.
Why Proper Pool Closing Matters
Failing to chemically prepare your pool for closure can lead to serious issues:
- Algae growth from neglected sanitization
- Corrosion or staining of the pool surface
- Chlorine lock or pH imbalance next season
- Poor circulation creating stagnant water zones
By using the right chemicals and preparing your pool correctly, you can protect both the structure and equipment from environmental insults and reduce the workload when opening your pool again.
The Essential Pool Closing Chemicals
Here’s a breakdown of the most critical pool closing chemicals and their role in preparing your pool for winter.
1. Chlorine (Shock Treatment)
Before you shut your pool down for the off-season, it’s crucial to perform a thorough shock treatment. The goal here is to eliminate bacteria, algae, and other contaminants.
Types of Shock You Can Use:
| Type | Pros | Cons |
|---|---|---|
| Calcium Hypochlorite | Fast-acting, affordable | Raises calcium hardness levels |
| Non-Chlorine Shock (MPS/Monopersulfate) | Safe for use before closing | No oxidation power |
| Dichlor or Trichlor | Stabilized, useful in sunlight | Should not be used as main winter shock |
2. Algaecide
Algaecide is critical when closing a pool to prevent algae growth during the stagnant water season. A quality algaecide ensures all potential spores are neutralized.
Choosing the Right Algaecide:
- Quaternary algaecides (Quats) are effective but can cause foamy water in the spring if overused
- Copper-based algaecides are long-lasting but must be used in precise quantities to avoid staining
- Polymer-based algaecides provide extended protection and are less likely to foam or stain the pool
3. Pool Shock (Clarifier or Flocculant)
While shock treatment eliminates pathogens, clarifiers or flocculants help bring all suspended particles to the bottom of the pool so they can be vacuumed up before closing.
Advantages:
- Removes microscopic particles that cloud water
- Improves vacuuming efficiency
- Minimizes the chance of discoloration or staining
4. Winterizing or Winterizer Chemical Kits
These kits often combine several essential chemicals like algaecide, shock, and pH balancers, all tailored for seasonal pool closure. They’re a convenient option for beginners and help ensure that you’re using the correct dosages for your pool size.
5. Enzyme-based Water Clarifiers
Modern pool owners are increasingly using enzyme products to break down non-living organic matter such as oils, lotions, and leaf debris. These organic chemicals do not replace standard sanitizers but enhance clarity and reduce chemical demand.
Step-by-Step: How to Use the Correct Chemicals When Closing Your Pool
Here’s a step-by-step process to chemically close your pool effectively:
Step 1: Test Pool Water
Before adding any chemicals, test your water using a reliable test kit or strip. Your water should maintain the proper chemical balance:
- Free chlorine: 1–3 ppm
- pH: 7.2–7.6
- Alkalinity: 80–120 ppm
- Calcium hardness: 200–400 ppm (adjust according to your pool type)
Use the test results to balance the water before moving to the next step.
Step 2: Shock the Pool Water
Select the appropriate shock product — most commonly calcium hypochlorite — and add at night for maximum efficacy. Shocking ensures any remaining microorganisms in the pool water are neutralized completely.
Step 3: Add Algaecide
Once the shock treatment has dissolved, apply a long-lasting winter algaecide. Choose a quat- or polymer-based formulation to reduce staining or foaming during spring opening.
Step 4: Add Clarifiers or Flocculants
If you choose a clarifier, it should be dosed after algaecide and run the pool pump for 3–6 hours before shutting it off. Alternatively, flocculant works faster and requires you to vacuum the debris the following day.
Step 5: Add Enzyme-based Clarifiers (Optional)
Enzyme products are particularly useful in soft-sided, above-ground pools or pools that may be prone to organic buildup over the off-season.
Step 6: Lower Water Level
Drain the water to about 4–6 inches below the skimmer, depending on your pool type and local winter temperatures. This prevents water from freezing and expanding in the plumbing.
Step 7: Apply Winter Chemicals According to Kit Instructions
Many pool supply stores offer pre-packaged closing chemical kits. Check the bottle instructions and apply according to square footage and volume.
Step 8: Winterize Equipment
Close off valves, add antifreeze to plumbing lines, and replace the filter’s water with an air pillow to prevent freezing damage. This step is particularly important for above-ground pools that are more sensitive to frost.
Common Mistakes to Avoid When Closing Your Pool
Avoiding these common errors ensures that your pool is chemically and physically protected from damage:
1. Skipping Shock Treatment
Unshocked water leads to bacterial and algae infestation. Even pools that looked clean at closing can become green in the spring if not properly shocked before covering.
2. Not Adding Enough Algaecide
Underdosing leaves behind algae spores. Be sure to use at least the recommended dosage (often 1 quart per 10,000 gallons) of algaecide during closure.
3. Adding Too Much Algaecide
Excess algaecide, particularly copper-based formulas, can stain your pool floor. Always follow the recommended use and distribute evenly around the perimeter.
4. Closing with Unbalanced pH
Water that is too alkaline, acidic, or hard can corrode or scale surfaces during the off-season.
5. Using Pool Covers Without Proper Preparation
The pool must be completely clean, chemically stable, and circulation stopped properly before applying a solid or mesh safety cover.
How to Store and Use Pool Chemicals Safely
To ensure your pool closing goes smoothly and safely:
Store Chemicals Properly
- Keep away from sunlight and moisture
- Store different chemical types apart
- Ensure lids are tightly sealed
Use Protective Gear
Always wear gloves, eye protection, and avoid inhaling fumes when handling pool chemicals.
Mix Only as Instructed
Never mix chemicals unless the manufacturer specifically allows it. For example, chlorine and acid can produce dangerous gases if improperly combined.
How to Choose Quality Pool Closing Chemical Products
Not all pool chemicals are created equal. Here are tips to help you select the best options:
Check for EPA Approval or Label Safety
Products should list active ingredients and include clear directions for winter use. Always check for manufacturer certifications and warnings.
Match to Your Pool Type
Saltwater pools, chlorine pools, above-ground pools, and vinyl pools all require slighly different treatments. For example, saltwater pool owners can use chlorine-free closing kits that combine clarifiers with UV protection and algaecide.
Use Trusted Brands
Some of the top pool closure chemical brands include:
| Brand | Main Product Type | Notes |
|---|---|---|
| Clorox Pool&spa | Algaecides, Shock | Popular for traditional chlorine users |
| Leslie’s | Pool Closing Kits | Trusted by DIY and professional owners |
| In The Swim | Flocculants, Clarifiers | Offers tailored seasonal kits per region |
| BioGuard | Enzymes, Algaecide | Top for enzyme-based clarifiers |
Winter Maintenance: Do’s and Don’ts of a Closed Pool
While closed, your pool still requires occasional attention:
Do:
- Check the cover tension and water level after storms
- Test water once a month if the winter is mild
Don’t:
- Allow snow or heavy rain to accumulate unevenly on the cover
- Open the pool cover unless it’s an adjustable mesh one
Conclusion: Closing Your Pool Like a Pro
A well-winterized pool is a stress-free pool come spring. By understanding what chemicals to use to close your pool — including shocking, balancing pH, adding algaecide, and using clarifiers — you can significantly enhance water quality and protect your investment.
Taking the time to close your pool properly not only extends the life of your pool surfaces and equipment but also saves time and money when you reopen. Whether you choose individual chemicals or a closing kit, be sure to make water testing a top priority before you store away your gear.
Final tip: Always label or store leftover pool chemicals together, ensuring they’re clearly marked and stored properly for next season. A small effort now pays big rewards when warm weather returns.
What chemicals are needed to properly close my pool for the winter?
To effectively close your pool for the winter, you’ll need several key chemicals to protect the water quality and the structure of the pool. These typically include a pool shock to eliminate bacteria and algae, algaecide to prevent algae growth during the off-season, a winterizing kit (which may include a combination of specialized chemicals), and a phosphate remover to reduce the food source for algae. Additionally, a chlorine neutralizer may be necessary if you’re using a non-chlorine shock, depending on your closing method.
Each chemical plays a vital role in winterizing the pool. Pool shock raises the chlorine level temporarily to sanitize thoroughly, while algaecide provides an extra layer of protection against algae that may survive the shock treatment. The phosphate remover ensures that no nutrients are left in the water to support algae growth, and the winterizing kit typically contains specially formulated products tailored to long-term water balance. Proper application and timing of these chemicals will ensure that your pool remains clean and clear throughout the colder months.
Why is it important to balance the water chemistry before closing the pool?
Balancing your pool’s water chemistry before closing is crucial to prevent damage and maintain water clarity during the winter months. If pH, alkalinity, or calcium hardness levels are out of range, the water can become corrosive or scale-forming, leading to damage to the pool surface, plumbing, or equipment. Balanced water also enhances the effectiveness of the winterizing chemicals, ensuring that algae and bacteria do not grow over the off-season.
Several parameters should be tested and adjusted before closing: pH should be between 7.2 and 7.6, total alkalinity between 80 and 120 ppm, and calcium hardness around 200–400 ppm depending on the pool type. Proper water balance also helps preserve the sanitizer residual throughout the winter, which can significantly reduce spring clean-up efforts. Skipping this step may result in costly repairs or extensive chemical treatments when you reopen the pool.
How much shock should I use when closing my pool?
The amount of shock needed to close your pool depends on the type of shock you’re using and the volume of water in your pool. For chlorine-based shock, such as calcium hypochlorite or sodium dichlor, a common recommendation is one pound of shock per 10,000 gallons of water. However, if your pool has a history of algae problems or the water is already cloudy, it’s advisable to double the dosage (2 pounds per 10,000 gallons) for more effective sanitation.
It’s important to apply the shock correctly: wait until evening or nighttime to minimize UV degradation, pre-dissolve the granules in a bucket of water if recommended by the manufacturer, and pour the solution around the perimeter of the pool while the pump is running. This ensures even distribution and effective oxidation of contaminants. Once shocked, wait at least 8–12 hours before applying other winterizing chemicals.
Do I need to use algaecide when closing the pool?
Yes, you should use algaecide when closing the pool as it serves as an important preventive measure against algae growth during the winter months. Algae can still grow, albeit slower, in cooler temperatures and dormant pools. By adding algaecide, you help ensure that your water remains clear and avoids the green or black slime that can be difficult to manage upon reopening.
The recommended dosage for most algaecides is 1 quart per 10,000 gallons of water, though you should always check the manufacturer’s instructions for specific guidance. It’s best to add algaecide after shocking the pool and before covering it for the season. Also, avoid using algaecides that contain copper, as they can cause staining if water chemistry is not properly maintained over the winter. A quality, non-staining algaecide is your best choice during pool closure.
Can I use regular chlorine tablets to winterize my pool?
Regular chlorine tablets are not sufficient on their own for winterizing a pool. While they can provide a consistent source of chlorine during the swimming season, they do not address all the necessary components required for winterizing. During pool closure, a more comprehensive approach is needed, which includes shocking the pool, balancing the pH and alkalinity, using algaecide, and possibly adding a winterizing chemical kit that’s formulated for extended protection.
Chlorine tablets may maintain residual chlorine levels during part of the off-season, but once water temperatures drop below 65°F, their effectiveness diminishes significantly. In addition, relying only on chlorine tablets can leave your pool vulnerable to algae and water balance issues. For a complete winter closing procedure, it’s best to combine shock, algaecide, proper water balance, and a winter cover to ensure the pool remains safe and clean until spring.
What is a phosphate remover and should I use it when closing the pool?
A phosphate remover is a chemical designed to eliminate phosphates from pool water. Phosphates serve as a nutrient source for algae, so removing them helps prevent algae growth during periods when the pool is not being actively maintained. While it’s not always required when closing a pool, using a phosphate remover is highly recommended, especially if you’ve struggled with algae in the past or if your pool receives a lot of organic debris like leaves and lawn runoff.
By reducing phosphate levels below 100 ppb (parts per billion), you significantly limit algae’s ability to grow and thrive over the winter. Phosphate removers typically contain lanthanum compounds or other binding agents that render phosphates inert. For best results, apply the phosphate remover early in the closing process, after balancing the water but before shocking and adding algaecide. This ensures that your pool water starts the winter season with the fewest possible nutrients for algae survival.
How do I know when to cover my pool after applying closing chemicals?
You should cover your pool only after all closing chemicals have been fully dissolved and circulated throughout the water. Typically, this takes about 8 to 12 hours after the last chemical addition. It’s important to circulate the water long enough to ensure even distribution of shock, algaecide, and any other treatments you’ve used. If you cover the pool too soon, some areas may not receive complete chemical coverage, leaving them prone to algae or bacterial growth.
In addition to allowing proper circulation, ensure that the water temperature is appropriate and that all debris has been removed from the pool. Once the chemicals are well dispersed, lower the water level according to your pool type (below the skimmer for in-ground pools), add the winter cover securely, and install an air pillow if you have an in-ground vinyl or concrete pool. Taking these final steps will help protect your pool structure and water quality over the winter season.